The manufacture of seamless cellulose tubes for sausage casings using cellulose derived by the so-called "viscous process" is well known in the art. Briefly, in the viscose process, a natural cellulose such as cotton linters or wood pulp is reacted chemically to form a cellulose derivative (cellulose xanthate ) which is soluble in a weak caustic solution. The solution or "viscose" is extruded as a tube into an acid bath. The extruded viscose reacts chemically with the acid bath resulting in the regeneration and coagulation of a pure cellulose tube. The chemical reaction produces several undesirable by-products including hydrogen sulfide and carbon disulfide.
More recently, a process of direct cellulose dissolution has been adapted to the manufacture of cellulose food casings. In this process no cellulose derivative is formed so the chemical reactions required first to form a cellulose derivative and then to regenerate the cellulose from the derivative have been eliminated. Instead, a natural cellulose is put directly into solution with the use of a tertiary amine oxide cellulose solvent such as N-methylmorpholine-N-oxide (NMMO). The resulting solution is thermoplastic in that it hardens upon cooling and flows on reheating. The solution, when molten, can be extruded as a tube or film into a water bath.
Upon contact with the water bath, the NMMO solvent is extracted so that a regeneration of the cellulose solution occurs. Thus, use of NMMO as a solvent for cellulose eliminates the need for derivatizing the cellulose, as in the viscose process. This avoids certain disadvantages of the viscose process such as the generation of gaseous sulfur compounds during the regeneration process. Moreover, the solvent is recoverable so it can be reused for cellulose dissolution.
U.S. Pat. Nos. 2,179,181; 4,145,532; 4,426,228 and Canadian Patent No. 1,171,615 all deal with the formation of a cellulose solution using the NMMO solvent and subsequentformation of cellulose articles such as films and filaments using the resulting solution. An apparatus and method for preparing an extrudable cellulose solution in a continuous process are disclosed in U.S. Pat. Nos. 5,094,690 and 5,330,567. In these patents, a suspension of cellulose in an aqueous solution of NMMO is fed into the top of a vessel having a heated wall. Within the vessel, a rotating wiper spreads the suspension across the heated wall and moves the suspension downward in the vessel. As the suspension moves downward, water is evaporated and the concentration of NMMO increases. Eventually, the temperature of the suspension and the concentration of NMMO reaches a level where the cellulose is dissolved so that a cellulose solution flows from the bottom of the vessel.
U.S. Pat. Nos. 5,277,857; 5,451,364 and 5,597,587 disclose a tubular extrusion method and apparatus utilizing the thermoplastic cellulose solution for purposes of making tubular films. Such films, for example, may be used as sausage casings.
As disclosed in these patents, the cellulose solution is extruded through an annular die and into a bath of nonsolvent liquid. Upon contact with the bath liquid, the cellulose solvent is extracted which causes the cellulose in the solution to be precipitated or "regenerated" in the form of a cellulose hydrate or "gel". The tube of cellulose gel is washed to remove residual solvent, plasticized and then dried to produce the tubular film of pure cellulose.
Due to economic factors, it is essential to recover for reuse, as much of the cellulose solvent as possible. Forthis reason, the aqueous solution from the precipitation bath and wash water is treated to evaporate or separate out the water in order to concentrate and recover the solvent.
In the production of tubular films, the process parameters which are found to produce desirable film characteristics are often compromised by process parameters for precipitating or regenerating the cellulose as fast as possible and for recovering the solvent.
For example, for purposes of rapid cellulose regeneration, a precipitation bath of hot water is preferred. With just water in the precipitation bath, solvent is extracted faster and a hot bath further accelerates extraction. However, due to the economics of solvent recovery, it is preferred to maintain the bath at a high solvent concentration. For example, at a solvent concentration of 5 to 30%, the bath still is considered a nonsolvent liquid for purposes of precipitating the cellulose but this concentration has the advantage of facilitating solvent recovery.
A second consideration for the precipitation bath is temperature. Generally a high temperature will accelerate the rate of cellulose precipitation by speeding the rate at which solvent is extracted from the solution. However, it has been found that desirable film properties are enhanced if the thermoplastic solution is extruded into a cool precipitation bath. It is possible the cool precipitation bath may promote the formation of a liquid crystal structure which improves the strength of the resulting film. The effect of the temperature of the precipitation bath on the properties of a tubular film are further set out in U.S. Pat. Nos. 5,658,525 and 5,658,524. Thus, conflicting desires of the economic solvent recovery and a strong film dictate that the precipitation bath be maintained at a solvent concentration of 10 to 25 to % and that the precipitated bath be relatively cold with a temperature of 0 to 8.degree. C. being preferred.
With the precipitation bath being maintained at a solvent concentration of 10 to 25%, not all of the solvent contained by the extruded solution is removed in the precipitation bath so that the regenerated cellulose gel contains an amount of recoverable solvent. Consequently, the film, while still in its gel state, is washed to strip out as much of the solvent as economically possible from the film. This adds to the economy of the method in that the solvent extracted can be recycled into the system to dissolve the natural cellulose for extrusion.
The washing conditions also may affect the characteristics of the resulting film. In particular, it has been found that subjecting the regenerated cellulose gel to a drastic temperature change adversely affects film properties. Thus, while washing the cellulose gel with hot water to speed solvent extraction is preferred, the cool cellulose gel leaving the precipitation bath should not be immediately exposed to hot wash water. Moreover, if the cellulose film is to be used for food contact applications, it is essential to reduce the residual solvent content of the film to levels well below any economic recovery level of the solvent and preferably to not more than 40 ppm. Thus, still further treatments of the cellulose gel are needed to satisfy governmental regulations for its use in food contact applications.
Accordingly, it is an object of the present invention to provide a process for regenerating a cellulose film from a solution of cellulose, a cellulose solvent and water including recovery of the cellulose solvent for reuse.
Another object is to provide such a process for regenerating a cellulose film which is suitable for direct food contact in that the level of solvent remaining in the film when ready for use is not more than 40 ppm on a dry cellulose basis.
A further object is to provide such a process wherein the regenerated cellulose gel is contacted with wash water at increasing temperatures to reduce the solvent levels in the regenerated cellulose gel to not more than 40 ppm.